Process for the stripping of actinides contained in organic solvents with nitrous acid



Dec. 19, 1967 A. BATHELLIER 3,359,079

PROCESS FOR THE STRIPPING OF ACTINIDES CONTAINED IN ORGANIC SOLVENTS WITH NITROUS ACID Filed Sept. 8, 1966 United States Patent 3,359,079 PROCESS FOR THE STRIPPING 0F ACTINIDES CONTAINED IN ORGANIC SOLVENTS WITH NITROUS ACID Andr Bathellier, Sceaux, Hauts-de-Seine, France, as-

signor to Commissariat a IEnergie Atomique, Paris,

France Filed Sept. 8, 1966, Ser. No. 577,879 Claims priority, application France, Sept. 17, 1965,

3 Claims. (C1.23 s39 ABSTRACT OF THE DISCLOSURE The present invention has for its object a process for stripping actinides and especially uranium, neptunium and plutonium from an organic tolvent into which they have been extracted by contacting their nitric acid solutions with said solvent.

The purification of uranium, neptunium and plutonium is usually obtained by solvent extraction processes. The solvents employed, such as tributylphosphate or trilaurylamine in solution in a parafiinic hydrocarbon such as dodecane, are loaded with the actinide by contacting with nitric acid solutions of these latter.

The following step of the process consists in stripping these metallic nitrates contained in the solvent. This stripping process can be performed by different methods such as the following:

Stripping by dilution-Since the metals are extracted into the solvent in the form of nitric acid complexes, it is merely necessary in theory to reduce the concentration of the nitrate ion in aqueous solution in order that the complex should be destroyed and that the metal should be no longer extractable. This method of stripping can be illustrated by the stripping of uranyl nitrate contained in tributylphosphate by dilute HNO Stripping by change of valence.-By means of a suitable oxidizing or reducing reagent, it is possible to change an extracted metal from an extractable valence state to an inextractable valence state. This is the case with nitric acid solutions of ferrous iron which reduce Pu IV to Pu III and cause this latter to leave the solvent which contains them.

Stripping by displacement.lf an acid has a high affinity for a solvent, it occupies all of the valence sites thereof and returns into aqueous solution the metals which were fixed therein. This mode of stripping can be illustrated by the stripping of plutonium nitrate contained in trilaurylamine by perchloric acid solutions.

Stripping by formation of c0mplexes.When any chemical substance in aqueous solution forms an inextractable complex with the extracted metal, its solutions are capable of re-extracting or stripping said metal. This is the case with stripping by sulphuric acid of the plutonium contained in trilaurylamine.

In practice, stripping processes are often of the composite type or, in other words, stripping by displacement is also a process of stripping by dilution and even by complexing.

The process in accordance with the invention consists ice in stripping the actinides and especially uranium, plutonium and neptunium which have been extracted into an organic phase consisting of tributylphosphate or trilaurylamine diluted in an inert solvent by nitrous acid.

Two observations have led the inventors to the development of the novel stripping process:

(1) The nitrous acid HNO has a sufiiciently high affinity for the usual organic solvents, namely tributylphosphate and trilaurylamine, to permit of stripping by displacement.

(2) Under predetermined conditions, nitrous acid is capable of reducing Pu IV to Pu III. The stripping action which is thus obtained is superimposed on the action which results from displacement.

Nitrous acid can be introduced into the stripping aqueous solutions in two ways:

Either in the form of sodium nitrite NaN-O in weak nitric acid solutions, in which case the nitrous acid is then unstable and dismutes slowly according to the reaction:

3HNO 2NO+HNO +H O (1) which shows that the HNO concentration is proportional to the power /3 of the pressure of NO.

The stripping yield by displacement evidently depends on the readiness with which nitrous acid leaves the aqueous phase to saturate the solvent. This readiness which varies from one solvent to another can be expressed in the form of distribution coefficients, viz:

org. (HNO aq.

The value of these coefficients has been determined by bubbling NO at atmospheric pressure in volume-to-volume mixtures of solvents and nitric acid having different concentrations. The results obtained are recorded in Table I below.

TABLE I HNOa, N in Aq. Sol. TB P, 30% TLA, 5% TLA, 20%

0. 05 1 2. 4 0. l 2. 5 0. 96 2. 3 0. 2 0. 74 2. 5 0. 3 7 0. 68 2 0. 5 7. 5 0. 61 l. 6 l 5. 4 0. 31 O. 94 1. 5 5. 5

It is apparent from a study of the above table that, so far as TLA is concerned, the increase in nitric acid concentration of the aqueous phase produces by competitive effect a reduction of the distribution coeflicient of nitrous acid. It is additionally found that nitrous acid has a special affinity for TBP.

In order to study the extent of the process of stripping of metals by displacement with nitrous acid, volume-tovolume mixtures of solvent loaded with uranium VI or neptunium IV and dilute nitric acid have been subjected for a period of three-quarters of an hour to a bubbling 3 of NO at atmospheric pressure; the a counting of separate phases has made it possible to establish the values of the distribution coefficients E of UO and N in respect of each type of mixture and to compare them with equal to 0.1 N. The portion shown in hatchings in the figure represents the zone in which it is reasonable to work in order to obtain the reduction of plutonium.

According to the process of the present invention, the

in respect of (NO) equal to 1 atmosphere and (NO the Values Obtained Without bubbling of NO. plutonium-loaded solvent is stirred in a nitric oxide atmos- TABLE II phere with dilute I INO the nitrous acid displaces a part of the plutonium rnto the aqueous phase in which it 1s with N0 Without NO then reduced. The equilibrium displacement then con- HNO1,N tlnues unt1l equ1l1'brat1on 1n aqueous phase of the oxidaaqueous Uvnnwz) Np 1V in U W in ND Iv in tion-reduction potentials of the pairs l?u u and TB}? TLA TBP 0% TLA NO3/NO. The final state of these equillbria 1s characterized by a distribution of the plutonium between the (125 organic and aqueous phase, which may be expressed by 0.2 0.24 0.7 o 0.3 0.42 1.3 the d1str1but1on coefficient E of plutonium. g :2 3' 15 A certain number of values of this coefiicient have been 1.5 10 determined by equilibrations which each lasted of an hour and are recorded in Table III hereunder.

TABLE III 30% TEP 20% TLA 5% TLA H1250; Aq. sol. HNO HNO; I-INO;

E1 Pu aq. EB Pu aq. Ea Pu aq.

equiL, M equiL, M equ11., M

Nitrous acid also makes it possible to re-extract pluto- Two examples of application of the process in accordnium from an organic phase by a change of valence. ance with the invention will now be described without Nitrous acid reduces Pu IV according to the reactions implied limitation, said examples being respectively apand in agreement with the following potentials: plied to the re-extraction of plutonium from an organic solvent constituted by trilaurylamine in a 5% solution by HNO IH O2NO -+3H 2e (4) volume in dodecane, and to the re-extraction of hexavalent uranium from an organic solvent consisting of 30% Pu4++e z pllg+ tributylphosphate in dodecane.

0 a EA=0i94+10g 0 058 (N 3 )(H Examplel I 2 (HNOz) The equipment employed cons1sted of a bank of mlxer- 4+ settlers which was completely enclosed and comprised 155:0'968 +1og 0058 twelve stages. The twelfth stage was reserved for the in- P11 troductron of the strrppmg aqueous phase and for the Th p0tentia1E hi h may b i t bubbling of NO in this latter, excess NO being then permitted to fill all the free sections of the bank and to be E1=0.94 +log 0.029 0.087 pH discharged at stage 1 through an outlet provided for this (H N02) purpose. shows h the pair NO3-/HNO2 is more highly reducing The solvent to be stripped which is admitted at stage 1 as the acid concentration of the solution is lower. has the followlilg COmPOSItIOIIi However, in order to avoid any danger of polymeriza- HNO 5% TLA in dodecane, Pu 0.375 gr./liter; tion and hydrolysis of the plutonium, the field of work is Free 0.05 N HNO at a flow rate of 4 v./hour. l1m1ted to a pI-l value wh1ch is lower than 1.4. The stripping aqueous solution consisting of 0.05 N In the case in Which the nrtrous ac1d 1s formed in solusaturated with NO is introduced in stage 11 at a tion by bubbling of NO, the concentration of this acid flow rate of 1 degends 1 of a glven On the Analyses carried out after equilibration of the bank of who NO/NO and t Same pp t0 the OXldatlOn mixer-settlers made it possible to draw up the following reductlofl P f table of concentrations of the different chemical species By ehmmating HNO between the two reactrons: employed T TABLE IV (8) l\O-|-H O HNO +H++e w1th Eo=0.99 v. and 4NO2, M HNOz, Pu, mgr./I

I Btank N 15. 3 -|-2e-HNO2+H2O Wlth V. 5 age Org. Aqq- Org. Aq' l1 There is obtained 7 NO+2H O+ NO -+4H++3e with :0.957 v. gggg 3 3g} 33;; gg 2 333 3-}; 0.047 0.061 0. 205 1; 380 0.10 the ox1dat1on-reduct1on potent1alofwh1ch may bewr1tten: 0. 04s 0. 064 0.181 50 960 0. 052 NO 3- 223 2- 3 3 3- 233 233 2- 23.

D 2.7 E =0.957+l0g 0.019 0.077 pH 0. 050 0.070 0.130 2 120 0.017

(NO) 2- 32 2-322 2- 22 -2 which is a decreasing linear function of the pH value. 01 04s 0:058 0Ii03 8:4 4 312 This function has been shown graphically in the figure 0040 3 These figures show that the stripping process or reextraction yield is 99.8%.

Example [I The equipment employed for the re-extraction of hexavalent uranium is identical with that of Example I.

The organic phase consists of 30% tributylphosphate in dodecane, and is loaded with uranium in a proportion of 72 g./l. (flow rate of 150 cm. /h.)

The aqueous phase has a nitric acid concentration of 0.125 N. During the manipulation, nitrogen dioxide is bubbled so as to introduce nitrate ions into the bank of mixer-settlers (fiow rate of 75 cm. /hr.).

The nitrogen dioxide is first washed with a concentrated sulphuric acid solution in order to eliminate the nitrogen peroxide. Its pressure within the 'bank of mixersettlers is adjusted to atmospheric.

After four hours of operation, the total acid concentration of the aqueous phase feed has increased from 0.125 N to 0.150 N.

The concentrations of the exit solutions are summarized in the following table:

Total acid con- Concentration Uranium (g./l.) Nitrate (M) centration (N) Organic 4. 5 0. 37 Aqueous 135 0. 09 0. 145

solution, it is merely necessary to increase the acid concentration. Accordingly, the nitrous acid re-oxidizes plutonium III to plutonium IV and the solution Which is obtained can be directly subjected to precipitation or to a further extraction process.

The stripping of U VI, Np IV, Pu IV contained in TBP or TLA is obtained by displacement at medium acid strength (1-1.5 N).

It is possible to separate Np IV from Pu IV by stripping.

In a nitric oxide atmosphere and in a 1 N nitric acid medium, the distribution coefiicients of Np IV and Pu IV in 20% TLA are respectively E =0.86 and E =7.3, and a countercurrent stripping under these conditions makes it possible to separate them.

What we claim is:

1. In a process for stripping actinides and especially uranium valence VI, plutonium valence IV and at a pH less than 1.4 and neptunium valence IV contained in organic solvents tributyl phosphate or trilaurylamine diluted in an inert solvent into Which they have been extracted by contacting their nitric acid solutions with said solvents, the step of stripping said actinides by nitrous acid.

2. A process as described in claim 1, said nitrous acid being formed in the stripping aqueous solution by the reaction of nitrogen dioxide with a dilute nitric acid solution out of contact with air.

3. A process as described in claim 1, said nitrous acid being formed in the stripping aqueous solution by reaction of sodium nitrite with a dilute nitric acid solution.

No references cited.

L. DEWAYNE RUTLEDGE, Primary Examiner.

BENJAMIN R. PADGETT, Examiner. S. TRAUB, R. L. GRUDZIECKI, Assistant Examiners. 

1. IN A PROCESS FOR STRIPPING ACTINIDES AND ESPECIALLY URANIUM VALENCE VI, PLUTONIUM VALENCE IV AND AT A PH LESS THAN 1.4 AND NEPTUNIUM VALENCE IV CONTAINED IN ORGANIC SOLVENTS TRIBUTYL PHOSPHATE OR TRILAURYLAMINE DILUTED IN AN INERT SOLVENT INTO WHICH THEY HAVE BEEN EXTRACTED BY CONTACTING THEIR NITRIC ACID SOLUTIONS WITH SAID SOLVENTS, THE STEP OF STRIPPING SAID ACTINIDES BY NITROUS ACID. 